This invention relates to a method and a system for controlling an induction motor fed from a frequency converting apparatus, more particularly to a method and a system for digitally controlling an induction motor.
When driving an induction motor (hereinafter merely called a motor) from frequency converting apparatus whose output voltage and frequency can be varied, the speed of the motor has been controlled by controlling the slip frequency thereof. The slip frequency has been controlled as a function of the primary current so as to improve the torque efficiency (the ratio of the output torque to the primary current) by taking into consideration the magnetic saturation of the motor, or the output torque has been made to be proportional to the primary current. These methods operate satisfactorily where the primary current is constant or varies gradually but when they are applied to a case where quick response is desired so that it is necessary to effect quick control, primary current transient oscillations occur in the output torque thus causing the control to become unstable.
It has been found that poor control response of the motor is caused by the fact that, since the frequency converting apparatus is constructed to supply the primary current to the motor, when the power factor varies due to the load variation of the motor, the phase of voltage with respect to current varies, in other words, the frequency of the magnetic flux varies thus causing unstable phenomena. These problems which occur when the torque of the motor is varied have been solved by varying the slip frequency, and the amplitude and phase of the primary current in accordance with the motor constants. As a consequence, a control response similar to that of DC motors can be provided for induction motors.
To effect such control, however, it is necessary to treat the primary current as a vector quantity involving not only its amplitude and frequency but also its phase. For this reason, it is necessary to arithmetically operate by considering also the motor constants so that the computation circuit becomes very complicated including a number of multipliers and dividers. Accordingly, to control the motor speed, including to a stop, it is necessary to control the frequency starting from zero so that it is difficult to precisely synthesize the slip frequency. Furthermore, adjustment of the computation circuit is troublesome. Where motors of a different rating or type are to be controlled, the operation must be adjusted. Even when the adjustment is made correctly, variation in the winding temperature and variation in the motor constants due to magnetic saturation prevent optimum control response, thus requiring a large amount of time and labor for accurate adjustment.